The stomach contents pass through the pyloric sphincter to the small intestine. A duct from the gall bladder adds bile, which breaks up fat droplets. This duct is joined by one from the pancreas which adds bicarbonate to neutralize the acid and adds enzymes which, along with those from the intestine's lining, complete digestion. Food is propelled through the digestive system by waves of muscular contraction called peristalsis. Once digestive enzymes have broken large food molecules into small ones such as onosaccharides, amino acids, and fatty acids, absorption of these nutrients can occur. The lining of the small intestine has a great many projections called villi. The surface cells of the villi have many small projections called microvilli. These greatly increase the surface area for absorption. Each villus contains capillaries and a lymph vessel. The products of fat digestion enter the lymph vessel. All other nutrients enter capillaries. Blood leaving the intestine flows directly to the liver in the hepatic portal vein. In the liver nutrients can be stored or converted to other molecules before release into the general circulation.